Titanium oxide-based sol-gel polymer

ABSTRACT

The invention relates to a titanium oxide-based polymer composition. The inventive composition comprises a TiO x (OH) y (H 2 O) z (x+y−+z=3) titanium oxide-based polymer in the form of a gel or sol. Said polymer, which has a one-dimensional (1D) structure, is made from concentrically-wound fibers having a periodicity which is deduced from the spacing between said fibers, of between 3.5 Å and 4 Å. Each fiber comprises TiO 6 octahedrons and each TiO 6 octahedron shares two opposite edges with two adjacent octahedrons (2.times.2.92 Å) in order to form infinite chains which develop along the axis of a fiber. According to the invention, two adjacent chains form double lines as a result of the shared edges (2.times.3.27 Å). The inventive polymer is suitable for use as a photosensitive element in a photovoltaic cell, such as a sunscreen for a window.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a division of U.S. patent application Ser.No. 10/502,399 filed Mar. 25, 2005, which is a national stage filing ofInternational Patent Application No. PCT/FR2003/00106 filed Jan. 14,2003, which claims the priority of French Patent Application No.02/01055 filed Jan. 29, 2002, the entire contents of which areincorporated herein by reference, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polymer composition based on titaniumoxide, to its use as a semiconductor element in a photovoltaic cell, andto a method for preparing it.

2. Description of the Related Art

Photovoltaic cells convert solar energy into electricity by exploitingthe photovoltaic effect that exists at the interface of a p-n junctionbetween two semiconductors. Semiconductors based on silicon have beenused, but the high cost of the raw material is not favorable to theindustrial development of such cells. Silicon has therefore beenreplaced with titanium oxide TiO₂ which is an inexpensive semiconductorand has stable photocatalytic properties. Its applications in thephotovoltaic field are, however, limited, as it absorbs only within anarrow range of the solar spectrum, owing to a wide bandgap. This rangecorresponds to the UV part and covers less than 10% of the entire solarspectrum. One solution consists in covering the surface of the titaniumoxide with a photosensitizer in order to extend its photoactivity rangeinto the region of the solar spectrum. This technique has been employedusing a ruthenium polypyridinic complex as photosensitizer (U.S. Pat.No. 5,084,365) and it has allowed efficiencies of around 12% to beachieved. The cells containing, as semiconductor, titanium oxideactivated by a photosensitizer have a production cost less than that ofthe photovoltaic cells of the prior art. However, their operatinglifetime, which is about 10 years, is considerably shorter than that ofsingle-crystal silicon cells (which is around 20 years), and theirefficiency is lower.

SUMMARY OF THE INVENTION

The inventors have now found that the performance of a titanium oxideused as semiconductor in a photovoltaic cell can be optimized bycontrolling the microstructural or mesostructural scale of themorphology. The object of the present invention is therefore to providea particular titanium oxide exhibiting improved performance when it isused as semiconductor element in a photovoltaic cell.

The subject of the present invention is therefore a composition based ontitanium oxide, a method for preparing it and a photovoltaic cell thatcontains it as semiconductor element.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

FIG. 1 shows the optical absorption spectrum of the untreated fractionof solutions 1 to 4;

FIG. 2 shows the optical absorption spectrum of the fraction of each ofsolutions 1 to 4 subjected to UV irradiation for 180 hours;

FIG. 3 shows the optical absorption spectrum of the fraction ofsolutions 1 to 4 subjected to heating at 65° C. for 15 hours;

FIG. 4 shows the optical absorption spectrum of the fraction ofsolutions 1 to 4 subjected to heating at 65° C. for 15 hours followed byUV irradiation for 15 hours;

FIG. 5 shows the idealized structure of a TiO(OH)₂ polymer ribbon;

FIG. 6 shows an absorption spectrum for a specimen using a CaryUV-Vis-NR absorption spectrometer; and

FIG. 7 shows an absorption spectrum for a specimen using a CaryUV-Vis-NR absorption spectrometer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The composition according to the present invention is essentially formedby a polymer based on titanium oxide, which may be represented by theformula TiO_(x)(OH)_(y)(H₂O)_(z) in which x+y+z=3, in the form of a gelor in the form of a sol. It is characterized in that:

the polymer has a structure of one-dimensional (1D) character and itconsists of fibers wound concentrically with a periodicity, deduced fromthe space in between the fibers, of between 3.5 Å and 4 Å;

each fiber is made up of TiO₆ octahedra;

each TiO₆ octahedron shares two opposed edges with two adjacentoctahedra (2×2.92 Å) in order to form infinite chains that grow alongthe axis of a fiber; and

two adjacent chains form double strands by the commoning of edges(2×3.27 Å).

The one-dimensional structure of the TiO_(x)(OH)_(y)(H₂O)_(z) polymer(denoted hereafter by TiO polymer) is detected by transmission electronmicroscopy. The chain structure is revealed by EXAFS analysis (ExtendedX-ray Absorption Fine Structure).

A polymer composition according to the invention (denoted hereafter bypolymer TiO composition) may be translucent or be colored. Thecomposition is translucent when it is shielded from the light and whenthe titanium is essentially in the Ti⁴⁺ form, the polymer thencorresponding to the formula TiO(OH)₂. When the titanium is essentiallyin the Ti³⁺ form in the TiO polymer, a broad absorption band exists inthe visible range (between 400 and 850 nm), which results in a violet,blue, midnight blue or green coloration of the composition. Thecoloration changes with the proportion of Ti³⁺. It goes from green, inthe case of low Ti³⁺ concentrations to blue in the case of highconcentrations. When all the titanium is in the Ti³⁺ form, the TiOpolymer corresponds to the formula TiO(OH)(H₂O).

A TiO polymer composition according to the invention may be obtainedfrom TiOCl₂. Since the TiOCl₂ compound is highly hydroscopic, it is usedin the TiOCl₂.yHCl form, i.e. in solution, dissolved in concentratedhydrochloric acid. Advantageously, the concentrated HCL solution is anapproximately 2M aqueous solution. The TiOCl₂ concentration in thissolution is preferably between 4M and 5M.

According to a first embodiment, the TiO polymer composition accordingto the invention may be obtained in oxidized form, in which the titaniumis in the Ti⁴⁺ oxidation state, by a method that consists in:

preparing a TiOCl₂ solution in dimethylformamide (DMF) by introducingTiOCl₂.yHCl into DMF, in proportions such that the concentration(C_(Ti)) of Ti atoms is less than 2M;

heating the solution thus obtained to a temperature between roomtemperature and 90° C.; and

holding the solution at this temperature for a certain time.

The temperature hold time depends on the temperature. For example, whenthe solution is held at 65° C., a time of 24 h is sufficient.

The TiO(OH)₂ polymer thus obtained may be converted into its reducedform in which at least part of the titanium is in the Ti³⁺ oxidationstate, by UV irradiation (for example at λ=360 nm) of the composition inan inert atmosphere, which induces a coloration (violet, blue or green,depending on the Ti³⁺ concentration), this coloration being maintainedwhen the irradiation ceases.

The TiO polymer composition is obtained in the form of a colloidalsolution or sol in DMF when C_(Ti) is less than 1M and in the form of agel when C_(Ti) is greater than 1M.

In a second embodiment, the TiO polymer composition of the invention isobtained directly in reduced form, in which at least a part of thetitanium is in the Ti³⁺ oxidation state by a method consisting inreducing TiOCl₂ using a species that is oxidizable at a potential ofless than −0.05 V with respect to a standard hydrogen electrode. As anexample, mention may be made of metals in oxidation state zero, such asNi, Fe, Al, Cr, Zr, Ti, Nb, Cs, Rb, Na, K, Li, La and Ce, ioniccompounds, in which the cation is chosen from V²⁺, Ti²⁺ and Cr²⁺, andionic compounds in which the anion is chosen from S₂O₃ ²⁻, H⁻, and S₂²⁻. Zinc is particularly preferred. In this case, the TiO polymercomposition according to the invention is obtained with a coloration. Ifit is then irradiated by UV radiation, the content of Ti³⁺ speciesincreases and its coloration changes from green to violet and then toblue as the content of Ti³⁺ ions increases.

A first variant of the method of preparation involving reduction by anoxidizable species, consists in preparing a TiOCl₂ solution indimethylformamide (DMF) starting with TiOCl₂.yHCl, such that theconcentration (C_(Ti)) of Ti atoms is less than 2M, in adding theoxidizable species, in heating the solution to a temperature betweenroom temperature and 90° C. and in holding the solution at thistemperature for a certain time, which depends on the temperature.

A second variant of the method of preparation, involving reduction by anoxidizable species, consists in introducing the oxidizable species intoa TiOCl₂.yHCl solution in which C_(Ti) is less than 2M, and inmaintaining the reaction mixture at a temperature between roomtemperature and 90° C.

In both variants, it is preferred to introduce the metal in the form ofchips. The ionic compound may be introduced in the form of powder,liquid or gas.

When a composition according to the invention is prepared by a methodusing DMF, it contains dimethylammonium chloride and formic acid. Theseconstituents may be detected for example by proton (¹H) NMR analysis,which also makes it possible to determine the quantity thereof. When theC_(Ti) concentration in the initial reaction mixture is less than 1M,the composition is a colloidal solution of uncrosslinked polymer in DMF.When the initial concentration C_(Ti) is greater than 1M, the polymer iscrosslinked and the composition is in gel form.

When a composition according to the invention is prepared according tothe second variant of the method involving reduction by an oxidizablespecies, i.e. in the absence of DMF, said composition is a colloidalsolution of uncrosslinked polymer in water when C_(Ti) is less than 1M.When C_(Ti) is greater than 1M, the polymer is crosslinked and thecomposition is in gel form.

Whatever the method used to obtain the reduced form of the polymerexhibiting coloration, the oxidized form may be obtained by subjectingthe polymer composition to oxidation in air so that it resumes itstranslucent appearance.

A TiO polymer composition according to the invention is photochromic incharacter. When it is obtained in gel form, it may advantageously beused in a photovoltaic cell in which the active material of thephotoanode is the composition containing the Ti³⁺ reduced form, and theactive material of the photocathode is a composition containing the Ti⁴⁺oxidized form.

A composition of the invention may furthermore be used for theproduction of solar protection glazing. A glass pane covered with acomposition of the invention in the form of a gel remains translucentwhen it is away from sunlight. Under the effect of irradiation byvisible light, the glass pane assumes a midnight blue coloration. Thisphenomenon can be made reversible by applying a potential allowingoxidation.

The present invention will be described in greater detail by thefollowing examples, which are given for illustration, but the inventionis not, however, limited thereto.

EXAMPLE 1

10 ml of DMF at 4 ml of a 4.3M TiOCl₂ solution in 2M hydrochloric acidwere introduced into a test tube, under an inert atmosphere of N₂. Afterhaving closed the tube, it was placed in an oven at 65° C. andmaintained at this temperature for 24 hours. It was then left to cooldown and the appearance of a transparent gel was observed at roomtemperature.

The presence of dimethylammonium chloride and formic acid was detectedby ¹H and ¹³C NMR by IR and by Raman.

After having been exposed to visible light, the gel had an intense bluecoloration, as a result of the reduction of Ti⁴⁺ to Ti³⁺. Thisphenomenon is reversible, and by opening the tube an oxidation takesplace in the presence of the oxygen from the air, the gel again becomingtransparent after a few minutes.

High-resolution imaging, obtained by transmission electron microscopy,showed that the structure of the TiO(OH)₂ polymer obtained was of1-dimensional (1D) character. The fibers of the polymer were woundconcentrically in the manner of a cotton bol. The presence ofsubstantial disorder in the direction perpendicular to the stack offibers was manifested in the diffraction pattern by the presence ofdiffuse lenticular spots. The periodicity, deduced from the spacingbetween the fibers, was estimated to be 3.5-4 Å.

EXAMPLE 2

The operating method described in example 1 was repeated, for severalpreparations, varying only the concentration C_(Ti) in the test tube.Each test tube, filled with air or N₂ was subjected to a heat treatmentsimilar to that of example 1.

The formation of a gel was observed only for C_(Ti) concentrationsbetween 1M and 2M. For concentrations where C_(Ti)<1M, the mixtureremained liquid and consisted of a colloidal solution of the polymer.For concentrations where C_(Ti)>2M, an opaque white product forms thatcontains a transparent polymeric phase and an amorphous whiteprecipitate, or a white precipitate of anatase in the case of very highconcentrations.

EXAMPLE 3

The optical properties of various specimens were measured for variousirradiation states. For this purpose, four specimens were prepared, inair or in nitrogen, from a TiOCl₂ solution identical to that used inexample 1:

No. C_(Ti) (mol/l) Vol. of TiOC1₂ Vol. of DMF O₂ N₂ 1 1.6M 1.3 ml 2.15ml X 2 1.6M 1.3 ml 2.15 ml X 3 1.45M 1.1 ml 2.15 ml X 4 1.45M 1.1 ml2.15 ml X

In a first series of tests, a fraction of each of solutions 1 to 4 wassubjected to UV irradiation (λ=360 nm) for 180 hours.

In a second series of tests, a fraction of each of solutions 1 to 4 wassubjected to a heat treatment at 65° C. for 15 hours, after which eachfraction was subjected to UV irradiation (λ=360 nm) for 180 hours.

FIGS. 1 to 4 show the optical absorption spectra of the solutions aftervarious treatments. The absorption A, in arbitrary units, is plotted onthe Y-axis. The wavelength λ, in nanometers, is plotted on the X-axis.In each of the figures, the solution spectra are indicated by thefollowing symbols:

Solution 1 Solution 2 Solution 3 Solution 4 ● ▴ ∘ Δ

FIG. 1 shows the optical absorption spectrum of the untreated fractionof each of solutions 1 to 4. The four spectra are almost identical andshow that there is no absorption in the visible range and that theinfluence of both the concentration and the conditioning atmosphere isnegligible.

FIG. 2 shows the optical absorption spectrum of the fraction of each ofsolutions 1 to 4 subjected to UV irradiation for 180 hours. The spectraindicate the presence of a strong absorption, which extends over a widerange in the visible, and also a slight shift of the absorption edgetoward shorter wavelengths.

FIG. 3 shows the optical absorption spectrum of the fraction of each ofsolutions 1 to 4 subjected to heating at 65° C. for 15 hours. Thespectra indicate a slight shift of the absorption edge toward shorterwavelengths relative to the spectra of the initial, untreated solutions.

FIG. 4 shows the optical absorption spectrum of the fraction of each ofsolutions 1 to 4 subjected to heating at 65° C. for 15 hours followed byUV irradiation for 15 hours. The spectra indicate a broad absorptionband in the visible range. The absorption is greater in the case of thegel and its maximum is shifted toward longer wavelengths than in thecase of the corresponding initial solutions.

The structure of the TiO(OH)₂ polymer gel was characterized by titaniumK-edge EXAFS analysis. The results of the fine structure analysis givethe number N of neighboring atoms, the distance R between an absorbentatom and its neighbors, the Debye-Waller factor sigma., the energy shiftΔE_(o) and the residue ρ. The results are given in the table below.

TiO(OH)₂ N R (Å) Σ × 10² (Å) ΔE_(O) (eV) P (%) Ti—O 3.91 1.89 1.3 0.48Ti—O 2.08 1.98 2.8 0.00 2.32 Ti—Ti 2.28 2.92 6.3 2.84 Ti—Ti 1.71 3.271.7 6.85

The idealized structure of the TiO(OH)₂ polymer ribbon, which has a 1Dcharacter, is shown in FIG. 5(B). It is similar to the structureobserved in the case of hollandite. Each TiO₆ octahedron shares twoopposed edges with two adjacent octahedra (2.92 Å) in order to forminfinite chains that run along the axis of the fiber. Two adjacentchains form double strands by commoning edges (2×3.27 Å). Because of thedifference between the actual number of neighbors and the ideal value of2, the polymer obtained may be crosslinked, as shown in FIG. 5(A).

EXAMPLE 4

Four starting solutions were prepared by introducing a TiOCl₂ solutionin concentrated HCl into DMF, in an amount such that the C_(Ti)concentrations were 0.03M, 0.04M, 0.05M and 0.06M respectively. 100 mgof zinc chips were added to 3 ml of each of these solutions.

The change in coloration was monitored over time by measuring theabsorption of the specimens using a Cary UV-Vis-NIR absorptionspectrometer between 300 and 1200 nm. The absorption spectra are shownin FIGS. 6 and 7. The absorption Abs. is plotted on the Y-axis (inarbitrary units). The wavelength λ is plotted on the X-axis (in nm). Thespectra show that, between t=0 min and t=500 min (FIG. 6), an absorptionpeak is formed at 550 nm, which increases over time and is accompaniedby three shoulders, at 630 nm, at 740 nm and at about 900 nm. After 500min (FIG. 7), this peak tends to disappear and leaves instead a broadabsorption band lying between 630 and 740 nm. After 3150 min, i.e. morethan 2 days, there is substantial absorption whatever the wavelength inthe visible range. However, two peaks appear at 550 nm and 710 nm.

1. A polymer composition essentially formed by a polymer based ontitanium oxide, which is represented by the formulaTiO_(x)(OH)_(y)(H₂O)_(z) in which x+y+z=3, in the form of a gel or inthe form of a sol, wherein: the polymer comprises fibers woundconcentrically with a periodicity, deduced from the space in between thefibers, of between 3.5 Å and 4 Å; each fiber is made up of TiO₆octahedra; each TiO₆ octahedron shares two opposed edges with twoadjacent octahedra in order to form chains that grow along the axis of afiber; two adjacent chains form double strands by the communing ofedges; and the polymer composition comprises dimethylammonium chloride(DMAC1) and formic acid.
 2. The polymer composition of claim 1, whereinthe polymer composition is translucent and contains titanium in oxidizedform Ti⁴⁺.
 3. The polymer composition of claim 1, wherein the polymercomposition has a violet, blue or green coloration and at least part ofthe titanium of the polymer is in Ti³⁺ form.
 4. A photovoltaic cellcomprising a photoanode and a photocathode in an electrolyte, wherein:the photoanode comprises a conductive glass plate coated with a layer ofthe polymer composition of claim 1 in gel form, containing titanium inTi³⁺ form; and the photocathode comprises a conductive glass platecoated with a layer of the polymer composition of claim 1 in gel formcontaining titanium in Ti⁴⁺ form.
 5. Solar protection glazing, whereinthe solar protection glazing comprises a glass plate covered with alayer of the polymer composition of claim 1 in the form of a gel.